Mineral indicators of geologically recent past life on Mars

Bioenergetic results of model simulations showing the feasibility of sulfate reduction, methanotrophy, methanogenesis and anaerobic methane oxidation (AMO) based on Gibbs energy scale. All reactions at equilibrium are exergonic at all model conditions. The Gibbs free energy reaction calculated in kJ/mol per electron transferred in a given reaction is shown. – Life

We provide new support for a habitable Martian subsurface microenvironment hosted in Fe- and Mg-rich rock units, and provide a list of minerals that may serve as indicators of specific water-rock reactions in recent geologic paleohabitats—under study.

Using a thermodynamic basis without selective phase suppression, we modeled the reactions of published Martian meteorites and Jezero Crater volcanic rock compositions and fair planetary water (saline, alkaline water) with Geochemist's Workbench Ver. 12.0 solid-phase inputs are meteorite compositions for ALH 77005, Nakhla and Chassigny, and two rock units from the Mars 2020 Perseverance rover sites Máaz and Séítah.

Six plausible types of Martian groundwater [NaClO4, Mg(ClO4)2, Ca(ClO4)2, Mg-Na2(ClO4)2, Ca-Na2(ClO4)2, Mg-Ca(ClO4)2] and a unique Martian soil-water analog solution (dilute saline solution) named „Rosy Red” associated with the Phoenix lander mission as aqueous inputs. Geophysical conditions are similar to the Martian near-surface (100 °C or 373.15 K, associated with residual heat from a magmatic system, impact event, or concentration of radionuclides, and 101.3 kPa, <10 m depth).

Mineral compositions were dominated by phyllosilicates such as serpentine-group minerals in most reaction pathways, but differed in some important indicator minerals. The sampled products differed in physicochemical properties (pH, Eh, conductivity), major ion activities and relative gas fugacities, with various environmental influences.

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Based on Gibbs free energy minimization, the microbial habitat of pore spaces in groundwater permeation systems at equilibrium in a thermodynamic framework was investigated. Models powered by the Chassigny meteorite produced higher H2 fugacity overall. Samples based on the Rosy Red soil-water analog produced the highest static CH4 fugacity (the highest values ​​were observed for reagent ALH 77005).

In general, the Chassigny meteorite protoliths produced excellent yields with respect to Gibbs free energy from an astronomical perspective. The occurrences of serpentine and saponite across the samples are significant: these minerals are observed using CRISM spectral data, and their formation by serpentinization is consistent with geologically recent-past H2 and CH4 production and sustained energy sources for microbial life.

We list indicator minerals used as diagnostic material for paleowater-rock samples that may support geologically recent past microbial activity, and recommend their use as benchmarks for future astronomical survey-site selections.

By Roger Hart and Don Cortes
Life 2023, 13(12), 2349; https://doi.org/10.3390/life13122349
Submission Received: 7 October 2023 / Revised: 25 November 2023 / Accepted: 5 December 2023 / Published: 15 December 2023


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